High-powered X-ray devices of the type used in such fields as medical diagnostics and X-ray crystallography require an anode capable of dissipating a relatively large amount of heat. Since the primary mode of dissipating this heat is by radiative heat transfer from the anode, an increase in the radiating surface area, leads to greater heat dissipation. By rotating the anode, a fresh area of the target surface can be continuously presented to the beam of electrons emitted by the cathode and the heat generated during X-ray production can be advantageously spread over a larger area. Thus, anode rotation allows an X-ray device to be operated at generally higher power levels than a stationary anode device and the problem of target surface degradation found in devices that use a stationary anode is avoided, provided the temperature limits of the target surface material are not exceeded.
The amount of heat generated and the temperatures achieved by an X-ray device can be substantial. Because less than as 0.5% of the energy of the electron beam is converted into X-rays while a major portion of the remaining energy emerges as heat, the average temperature of the target surface of the rotatable anode can exceed 1200.degree. C. with peak hot spot temperatures being substantially higher. The reduction of these temperatures and dissipation of the heat is critical to any increase in power. The ability to dissipate the generated heat by anode rotation alone, however, is nonetheless limited. As a consequence, even though there has been a demand for ever higher-powered devices since rotatable anodes were first introduced, the development of such devices has lagged.
A further disadvantage of prior art devices is their limited lifetime, which is determined in part by their ability to dissipate heat. Since X-ray devices can be relatively expensive, extending the lifetime of such a device will result in substantial cost savings.
In an X-ray device, it is primarily the bearings on which the anode shaft rotates, which determine the devices lifetime. The bearings used with a rotating anode are typically placed within the evacuated glass envelope to avoid the need for a rotating vacuum seal. Placing the bearings in this vacuum, however, necessitates the use of special lubrication, e.g. a silver coating placed on the bearing, which can itself be heat sensitive. The temperature of the bearings can at times exceed 400.degree. C., due primarily to the conduction of heat from the anode through the shaft on which the anode turns and thereby into the bearings. Thus, a heat intensive hostile environment is created that can quickly result in the erosion of the bearings, leading to seizing of the shaft and ultimately to failure of the device.
Proper cooling in order to maintain the bearings of the X-ray device below a critical temperature of about 400.degree. C. will advantageously extend the lifetime of the bearings and, hence, of the device itself. Such cooling is further desirable because it permits an increase in the peak and average power levels over and above that of existing X-ray devices, thus extending the capability and utility of such devices over those in use today.
The time averaged heat dissipation of the X-ray tube used in a CT scanner determines the patient throughput. It is estimated that the required average energy output of the pulsed electron beam is 12 kw. Present day CT scanner tubes dissipate approximately 3 kw. When the target of the X-ray tube overheats, as will happen if patient throughput is increased, the time between subsequent uses of the machine will have to be increased to allow the target to cool. An X-ray tube with higher dissipation will allow improved machine utilization.
Typical of prior art attempts at cooling X-ray devices is U.S. Pat. No. 4,455,504 to Iversen. As disclosed in this patent, cooling occurs by circulation of a fluid through the anode interior in direct contact with the interior surfaces of the anode. While such a system promotes cooling, it necessitates the use of rotary fluid seals. Since the seals are prone to leak, the reliability of such a device may be low and there is no assurance that the device will survive such a leak if it occurs.
It is an object of the present invention to provide a new and improved X-ray generating device that is not subject to the foregoing disadvantages.
It is another object of the present invention to provide a new and improved high-powered X-ray device which has a longer useful life than heretofore available devices of this type.
It is still another object of the present invention to provide a new and improved X-ray device having an increased heat dissipation rate which will permit continuous operation.